I actually tested some more and it came down that it only did this with a specific mail server (smtp.1and1.com), the same code works just fine now with Postfix on localhost, I tried it not just with sendmail, but also the smtp adapter and it's okay. However zf1 type mail handling worked fine with 1and1 mail servers, so there must be something that fails during smtp communication with zf2 and those servers.

Posted by Philipp Kamps (fips) on 2012-05-01T13:30:18.000+0000

I can confirm that there are some problems around sending emails over different mail servers. Sometimes I end up receiving correct emails and sometimes (on different networks with a different mail delivery setups) I only receive broken messages. Here an example: